1. Field of the Invention
The present invention relates to an electron emitting element that emits electrons in response to application of a voltage; and a method for producing the electron emitting element.
2. Description of the Related Art
Electron emitting elements comprising a Spindt-type electrode and a carbon nanotube (CNT) electrode are known as conventional electron emitting elements. Applications of such conventional electron emitting elements to, for example, the field of Field Emission Display (FED) have been studied. Such electron emitting elements are caused to emit electrons by tunnel effect resulting from an intense electric field of approximately 1 GV/m that is formed by application of a voltage to a pointed section.
However, these two types of electron emitting elements have the intense electric field in the vicinity of a surface of an electron emitting section. Accordingly, electrons emitted obtain a large amount of energy due to the electric field to be more likely to ionize gas molecules. Cations generated due to the ionization of gas molecules are accelerated toward and collide with a surface of the element due to the intense electric field. This causes a problem of breakdown of the element due to sputtering. Further, ozone is generated before ions are generated, because oxygen in the atmosphere has dissociation energy that is lower than ionization energy. Ozone is harmful to human bodies and oxidizes various substances because of its strong oxidizing power. This causes a problem in that members around the element are damaged. In order to prevent this problem, the members around the element are limited to materials having high resistance to ozone.
With such background, MIM (Metal Insulator Metal) type and MIS (Metal Insulator Semiconductor) type electron emitting elements have been developed as other types of electron emitting elements. These electron emitting elements are surface-emission-type electron emitting elements, which accelerate electrons by utilizing quantum size effect and an intense electric field in the element so that electrons are emitted from a flat surface of the element. These electron emitting elements do not require an intense electric field outside the elements, because the electrons accelerated in an electron acceleration layer in the elements are emitted to the outside. The MIM type and the MIS type electron emitting elements can therefore overcome the problem of breakdown of the element by sputtering due to ionization of gas molecules and the problem of ozone generation, which are likely in the Spindt-type, CNT type, and BN type electron emitting elements.
However, such electron emitting elements are generally prone to pin holes or dielectric breakdown. Against this problem, there is a known technique to prevent the pinholes and the dielectric breakdown by using an insulating film having fine particles in such electron emitting elements. For example, an MIM type electron emitting element provided with an insulator containing fine particles between two sheets of electrodes opposed to each other is known (see Japanese Unexamined Patent Publication No. HEI 1(1989)-298623, for example). In addition, an electron emitting element is known, comprising a carbon nanotube electrode in which an insulating film formed of a powder layer composed of insulating particles and a fixed layer composed of an oxide insulator and formed so as to cover the powder layer is disposed between an electron emitting section formed of a carbon-based electron emitting material and an electron extraction electrode disposed on the electron emitting section for extracting electrons from the electron emitting section (see Japanese Unexamined Patent Publication No. 2000-311640, for example).
While these electron emitting elements have an insulating film as its component, however, the insulating film may cause decrease in the amount of electrons being emitted from the electron emitting element in some cases where the film is so thick that the electric resistance thereof increases. It is therefore necessary to apply a higher voltage to the electron emitting element; so development of an electron emitting element that can emit a sufficient amount of electrons by application of a moderate voltage has been desired. In addition, the insulating film may shorten the time for the electron emitting element to continuously operate in some cases where the film is so thin that it is difficult to form a uniform insulating film and a dielectric breakdown is more likely to occur. Development of an electron emitting element that can continuously operate for a longer time has been therefore desired.